The present invention relates generally to electronic ballasts that include control circuitry.
More particularly, this invention pertains to power supply circuits and methods for supplying stable power to control circuitry included in electronic ballasts.
Electronic ballasts that include control circuitry are known in the art. These devices typically include an AC/DC converter circuit, a power factor control (PFC) circuit including a PFC control chip, an inverter circuit, which includes an inverter gate drive chip, and a microcontroller. The control circuitry includes the PFC control chip, the inverter gate drive chip, and the microcontroller. The lossless power supply management circuitry is designed to ensure that these chips receive appropriate bias supply at all times.
The AC/DC converter is designed to convert low frequency AC voltage from an AC power source into rectified AC voltage and the PFC circuit is designed to convert the rectified AC voltage into an approximately constant DC voltage and to cause power drawn from the AC power source to have a desired power factor. The inverter circuit is designed to convert the approximately constant DC voltage into high frequency sinusoidal AC voltage and to use that voltage to supply current to a gas discharge lamp connected to the electronic ballast. When the electronic ballast is operating normally, the inverter circuit loads the PFC circuit and the gas discharge lamp loads the inverter circuit.
The microcontroller is responsible for controlling the inverter circuit and causing it to generate the high frequency sinusoidal AC voltage supplied to the gas discharge lamp. In addition to controlling the inverter circuit, the microcontroller is also usually capable of automatically detecting and igniting lamps that are connected to the electronic ballast and protecting the electronic ballast and lamps from being damaged by fault conditions occurring in the electronic ballast.
The microcontroller, the PFC control chip, and the inverter gate drive chip included in the control circuitry of prior art electronic ballasts are all digital control circuits that have relatively strict power supply requirements. Unlike analog integrated circuits, which can continue to operate properly if their input voltage drops briefly, these digital circuits will not continue to operate properly if their input voltage drops too low. This is particularly true for the microcontroller, which will reset and restart as if the electronic ballast had just been turned on if its input voltage drops below a certain level.
The prior art teaches the use of three different types of power supply circuits and methods to supply power to control circuitry in electronic ballasts: the starting power supply circuit, the PFC power supply circuit, and the inverter power supply circuit. However using these circuits to provide power to the chips when the ballast is shut down either in response to a dimming off command or while waiting for a replacement lamp will either waste power or have other side effects such as producing undesired output voltages. In addition, the starting power supply circuit consumes unnecessary power, which reduces the efficiency of the ballast, and the alternative of limiting the amount of power consumed by it then causes electronic ballasts using this type of circuit to start slowly.
An example of a prior art starting power supply circuit is shown in FIG. 1 supplying power to a PFC control chip and includes a resistor R1 connected to the high DC voltage input of the PFC circuit, a capacitor C3 connected to the resistor R1, a Zener diode D8 connected across the capacitor C3, and a blocking diode D7 connected to the Zener diode D8. The resistor R1 converts the high DC voltage input into a starting current that is supplied to the PFC control chip and used to charge the capacitor C3, which generates the required starting voltage for the PFC circuit. The Zener diode D8 prevents the starting voltage from exceeding a maximum starting voltage level and protects the PFC control chip from excessively high input voltages, and the blocking diode D7 prevents current from flowing back into the voltage regulator circuit included in the PFC power supply circuit.
Due to high power losses generated by the resistor R1, this type of circuit only provides a very small fraction of the power required by the control circuitry to operate properly and is typically only used to supply the PFC control chip with power until a PFC power supply circuit or an inverter power supply circuit can do so. In addition, once the PFC or the inverter power supply circuit begins supplying power to the PFC control chip, the starting power supply circuit continues to consume power even though it is no longer needed. This increases the amount of power consumed by, and reduces the efficiency of, electronic ballasts using this type of circuit. The high resistance of the resistor R1 also limits the amount of current that can flow in the circuit and increases the time required to start the PFC control chip.
An example of a prior art PFC power supply circuit is also shown in FIG. 1. This type of power supply circuit is widely used in the prior art and can be used to supply power to all of the electronic ballast control circuitry.
The PFC power supply circuit includes an auxiliary winding connected to a boost inductor L1, a charge pump connected to the auxiliary winding that includes R3, C2, D5, D6, and C4, and a voltage regulator circuit connected to the charge pump that includes the voltage regulator chip U2. The auxiliary winding generates and supplies a low voltage rectified AC voltage to the charge pump (the auxiliary winding is also used to provide zero current detection for transient mode operation of the PFC circuit), the charge pump uses this voltage to generate an input voltage that is supplied to the voltage regulator circuit, and the voltage regulator circuit uses the input voltage to generate and supply the required operating power to the control circuitry. More specifically, the voltage regulator circuit supplies a regulated +5 volt voltage to the microcontroller circuit and a +15 volt voltage to the PFC control chip and the inverter gate drive chip. The connection between the PFC power supply circuit and the inverter gate drive chip is not shown in FIG. 1 in order to simplify that drawing.
When the PFC circuit shown in FIG. 1 is operating normally, that is, it is converting the rectified AC voltage supplied by the AC/DC converter circuit into the approximately constant DC voltage that is supplied to the inverter circuit, the PFC power supply circuit is capable of generating and supplying the operating power required by the control circuitry in the electronic ballast. When the PFC circuit is not operating normally, however, the voltages generated by this type of power supply circuit drop out and cannot be used to supply the required operating power to the control circuitry. As explained in more detail below, this typically occurs when the inverter circuit is not loading, i.e., drawing current from, the PFC circuit.
The prior art solution to the problem presented by the PFC power supply circuit is to connect a load resistor (R9 in FIG. 1) across the bulk capacitors included with the PFC circuit. The load resistor provides a load to the PFC circuit and causes it to remain active even when the inverter circuit is not running. While this solution does work, it reduces the efficiency of, and generates extra heat in, the electronic ballast because the load resistor constantly consumes power. For a four-lamp ballast, the typical loss on the load resistor is approximately 2.7 watts.
An example of a prior art inverter power supply circuit is shown in FIG. 4. As shown in that figure, the inverter power supply circuit is connected to a midpoint between the two power MOSFET transistors included with the inverter circuit. The inverter power supply circuit includes a charge pump and a voltage regulator circuit, both of which are similar to the charge pump and voltage regulator circuits shown in FIG. 1. The inverter power supply circuit does not include the auxiliary winding connected to the boost inductor used with the PFC power supply circuit.
When the inverter circuit shown in FIG. 4 is operating normally and converting the approximately constant DC voltage supplied by the PFC circuit into high frequency sinusoidal AC voltage and current that is supplied to the gas discharge lamp, the inverter power supply circuit is capable of generating and supplying operating power to the control circuitry. If the inverter circuit stops running for some reason, this power supply stops as well and the required operating voltages for the control circuitry drop out. The prior art does not teach a solution for this problem.
As a result of the disadvantages discussed above, the PFC and inverter power supply circuits discussed above cannot be used to supply stable power in an electronic ballast that includes a microcontroller that provides advanced functions which require prolonged periods of inactivity, such as when the ballast has shut down because a lamp has reached end of life and the ballast is waiting for relamping. When the microcontroller in this type of electronic ballast senses that the ballast has been connected to a gas discharge lamp, it automatically causes the inverter circuit to attempt to ignite the lamp by turning the inverter circuit off and on for brief periods of time, i.e., flashing the lamp. More specifically, the microcontroller turns the inverter circuit off and on every two seconds for a 100 second time period causing the lamp to flash 50 times. During each 2 second time interval, the inverter circuit is on for approximately 10 milliseconds and off for the remainder of the interval. The duration of the flashes and the intervals between flashes are set by industrial regulations, such as Underwriters Laboratories (UL) regulation UL 935, and are designed to prevent the electronic ballast from presenting an electric shock hazard to a human being.
Unfortunately, the limitations on the flashing duration and interval make it impossible to feed current from the inverter circuit to the lamp during the flashing period. As a result, the inverter circuit turns off and no longer loads the bulk capacitors included in the PFC circuit. This, in turn, causes the PFC circuit to turn off.
When the inverter circuit turns off, the charge pump in the inverter power supply circuit turns off and the voltages generated by the inverter power supply circuit drop too low to be used to supply the required voltages to the control circuitry in the electronic ballast. In a similar manner, when the PFC circuit turns off, the charge pump in the PFC power supply circuit turns off and the voltages generated by the PFC power supply circuit drop too low to be used to supply power to the electronic ballast control circuitry. Thus, neither of these types of power supply circuits can be used in electronic ballasts providing this type of advanced functionality.
What is needed, then, is a starting power supply circuit and method that can be used to supply starting power to a PFC control chip in an electronic ballast more quickly and that consumes less power and is more efficient than prior art starting power supply circuits. In addition, what is needed is PFC and inverter power supply circuits and methods that can be used to supply stable power to the control circuitry even when the ballast output is completely shut down.